FSQ focuser tuning. Here are a few images showing the bearings: Linear bearings mounted to a 44cm (~17 inch) dovetail plate

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Austin Wood
6 years ago
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FSQ focuser tuning Few months ago, I bought a second-hand FSQ-106ED. I was aware of discussions that the focuser of the FSQ does not match the optical quality of the scope.

1 FSQ focuser tuning Few months ago, I bought a second-hand FSQ-106ED. I was aware of discussions that the focuser of the FSQ does not match the optical quality of the scope. There are several reports indicating that the focuser flexes when loaded with state-of-the-art heavy imaging gear. My setup consists of a FLI ML8300 and the FLI CFW-2-7 filterwheel with Baader LRGB and smallband filters. This setup weights about 3.5 kg (~7 lbs), therefore it s not leightweight but also by far not the most heavy setup used today. After putting the image train together, it became very obvious that there is a noticeable flex in the focuser. By putting moderate pressure on the filterwheel or CCD, I could easily see some movement. I also did a first rough measurement by adding 1,2 kgs of weight on top of the CCD. This caused a shift of the CCD of about 0,15 mm. Further measurements indicated that the majority of the flex is caused by the focuser and not by any of the components between the focuser and the CCD. The captain wheel of the FSQ is also considered to introduce flex, so I screwed it tight as much as possible. As flex in the image train is something that can damage collimation (especially when using the FSQ with the reducer) and also causes undesirable issues when creating pointing-files with my ASA DDM60 Pro mount, I decided to do something I did a year ago with my TMB80/480. In order to prevent the CCD from moving relative to the optical axis I build a rig based on linear bearings that supports the CCD camera and still allows for focusing. Linear bearings are similar to roller bearings but instead of turning, they slide on a rail. They can carry heavy loads, have very little friction and hardly any play in the two axis vertical to the rails. I found two suitable linear bearings left from the days when I built my own CNC mill. I mounted the bearings on the longest dovetail plate I had. On top of the two bearings sits a block of the right height to support the CCD. The CCD gets attached to the block using a ¼ inch screw. The setup does not allow the CCD to be rotated anymore, but that s ok for me at the moment. In case I need to change that, I might support the filterwheel instead of the CCD and rotate the CCD in the filterwheel. Here are a few images showing the bearings: Linear bearings mounted to a 44cm (~17 inch) dovetail plate

2 Linear bearings with the mounting block to support the CCD CCD attached

3 The whole setup The difference in terms of flex is very noticeable. Putting moderate pressure to the CCD does no longer show any signs of flex. In order to verify the difference before and after the modification, I fixed a bracket for a digital dial gauge to the dovetail (see next picture). The tip of the dial gauge rests at the side housing of the CCD. That way I could tilt the image train in different angles, measure the flex and compare the results with the CCD screwed down to the supporting block vs. not being fixed.

I put the whole image train on a Giro-mount and started with the scope being

4 Digital dial gauge To verify the difference in flex, I started with the CCD firmly screwed down to the supporting block. I put the whole image train on a Giro-mount and started with the scope being horizontal to the left (I ll call that position west ). At that position, I zeroed the dial gauge. Starting position west

5 Then I moved the scope to the zenith position. The dial gauge still displayed 0.00 mm (last digit is 0.01mm). Zenith position And finally to the opposite horizontal position east.

East position Here, the dial gauge read 0.01mm. That looked very promising, especially considering the fact that the east and west positions are not very commonly used when snapping astro photos.

6 East position Here, the dial gauge read 0.01mm. That looked very promising, especially considering the fact that the east and west positions are not very commonly used when snapping astro photos. Now I did the same test with the CCD not being attached to the supporting block. The difference between the east and the west position was 0.89mm! That s an equivalent of nearly 165 pixels on the chip of the ML8300. That s a lot of flex given the fact that the FSQ focuser is fairly nice build. That amount of flex creates a few challenges: Even though it might easily be compensated by off-axis-guiding, it s a real issue when using a guide-scope or as in my case no guiding at all (that s what I bought the DDM60 for in the first place). Given the very small critical focus zone of the FSQ, especially in the configuration with the reducer, collimation will get lost as the flex basically tilts the CCD away from the image plane, leading to unsharp/distorted stars in the corners. The flex clearly shows some hysteresis and sagging, meaning that the flex is different depending of what direction you approach a position from. In my case moving the scope from east to zenith then to west and back to zenith showed a difference in the zenith position flex of 0,12 mm which slowly was reduced to about 0.05mm after sitting in the zenith position for about one minute. This is a very bad situation for the creation of pointing files and for pointing accuracy in general.

7 One might think that the biggest amount of flex is only introduced once the scope gets close to the horizontal positions where one would not image anyway. So I did a test starting at the zenith position and then measuring the flex every 15 degrees going towards the horizontal position. The results were the following: Position [degree] Flex [mm] compared to Zenith position 0 (zenith) (horizontal) 0.44 So it s obvious that even in case one stays above 45 degrees, there s a significant flex. With the ML8300 that might mean that if you are not doing off-axis-guiding and want to keep the error introduced by flex under 2 arcseconds, the maximum exposure time is about 4.5 minutes. There was a lot of discussion in the uncensored Tak yahoo group as well as in the ASA yahoo group with regards whether or not the FSQ focuser is adequate for todays imaging or whether flex in the image train causes problems going unguided. For me it is pretty obvious, that with todays CCD-equipment, the focuser is more and more getting the weak point in the imaging train. Therefore several astrophoto enthusiasts replace stock focusers by e.g. FLI Atlas or the big Feathertouch focusers. I plan to do a similar test with my 3 and 3,5 Feathertouch focusers soon.

Using an Astrodon MonsterMOAG with a Takahashi FSQ-106N Frank Barnes III December 15, 2007

Using an Astrodon MonsterMOAG with a Takahashi FSQ-106N Frank Barnes III http://www.skyimager.com December 15, 2007 Taking guided CCD images with a telescope/camera combination presents unique challenges